Prestressing pump fluid ends



Spt. 5, 1967 D. R. WILSON ETAL PRESTRESSING: PUMP FLUID ENDS Filed Nov. 6. 1964 00 P. W/AS ON 7 MLL/fiM K MFODOX,

BY am, pm, MID-J 1 m ATTORNELS 4 INV EN TOR5 United States Patent 3,339,264 PRESTRESSING PUMP FLUID ENDS Don R. Wilson and William K. Maddox, Duncan, Okla., assignors to Halliburton Company, Duncan, 0kla., a corporation of Delaware Filed Nov. 6, 1964, Ser. No. 409,539 Claims. (Cl. 29-407) This invention relates to a method of prestressing hollow vessels, and more particularly, to a method of prestressing at very high pressures.

Prestressing of the interior of hollow vessels, such as valve bodies and pump cylinders, has been accomplished in accordance with the teachings of the prior art by sealing the outlet openings of the hollow vessel and injecting a fluid into the interior of the vessel. The pressure of the fluid is increased to a value substantially greater than the working pressure expected to be imposed on the interior of the vessel in regular service. The internal fluid pressure in the vessel induces stresses in the vessel which exceed the yield point of the metal during prestressing, so that a permanent set occurs. The compressive stresses in the inner surface of the vessel are balanced by tensile stresses in the outer surface, and accordingly, the working strength of the hollow body is substantially increased. United States Patent No. 2,117,351 describes this method of hydraulically prestressing a valve body.

In recent years, fluid operating pressures have increased considerably, and it is necessary to prestress hollow vessels at substantially higher pressures. There is a practical limit to the pressures which may be imposed when conventional prestressing fluids are utilized. Many of the common fluids used for prestressing solidify at lower pressures than are necessary to do the work. Water may be used at higher pressures than oil, but it does not remain in the liquid state at very high pressures, which may be encountered in prestressing.

As the internal prestressing pressures for vessels have become higher, the danger of explosion of the vessel presents another serious problem. Conventional fluids used for prestressing are compressible. Some oils, for example, may have compressibilities of over at 50,000 psi. As the interior surface of .the body expands, the hydraulic pressure remains substantially the same. If the expansion of the metal causes weakening of one portion of the hollow body, then the pressure of the fluid may be sufficiently great to cause the hollow vessel to burst. For this reason, prestressing with compressible hydraulic fluid at high pressures is extremely hazardous.

In view of the defects of prior art methods of prestressing hollow vessels, it is an object of this invention to provide a method for prestressing hollow vessels at very high internal pressure.

It is a further object of this invention to provide an efficient method of prestressing hollow vessels.

It is another object of this invention to provide for high pressure prestressing of hollow vessels without the danger of exploding the vessel.

These objects are accomplished in accordance with a preferred embodiment of the invention by plugging or otherwise temporarily closing the outlet openings in a hollow metal vessel, filling the interior of the vessel with a low melting temperature metal of the type having a plastic solid phase, and displacing the metal outwardly against the interior surface of the vessel. The metal remains in a solid phase in the vessel during the prestressing operation. A plunger or ram passes through a sealed collar and projects into the body of metal which fills the interior of the vessel. Displacement of the ram further into the vessel causes the solid metal to flow plastically until it completely fills the interior of the vessel and the internal pressure of the metal increases. The force-on the ram is ap- 3,339,264 Patented Sept. 5, 1967 proximately proportional to the hydraulic pressure in the metal, and since the metal is substantially incompressible, the yielding of the internal surfaces of the vessel causes the hydraulic pressure to be partially relieved. The eX- pansion of the interior surface beyond the yield point of the metal in the vessel is continued until the desired prestressing pressure is reached. The operation can be controlled to the desired pressure by measuring the amount of yield which has been obtained.

This preferred embodiment is illustrated in the accompanying drawing which is a cross sectional view of a conventional fluid pump cylinder block which is being prestressed in accordance with this invention.

In hydraulic pumps, the cylinder walls, piston and cylinder head are exposed to cyclic loading and over a period of time metal fatigue often causes these parts to fracture, but prestressing substantially lengthens their service life. The prestressing method of this invention is described as carried out on a cylinder block 1, as shown in the drawing. The cylinder block 1 has a shoulder 2 machined in the inlet passage 3, in the outlet passage 4 and in the rough cylinder opening 5. Metal plugs 6 are mounted on the respective shoulders 2 to form a closed cavity 7 in the cylinder block 1. On the opposite side of the block 1 from the piston opening 5, screw threads 8 are machined in an opening 9 in the head 1. Any other suitable arrangement can also be used to retain a cover on the opening 9. The opening 9 will be occupied by a conventional cylinder head when the pump is assembled for operation. The cavity 7 is filled full of liquid metal 10, such as melted lead, after a sealing ring 11 has been inserted in the opening 9, and a packing retainer 12 has been secured over the opening 9. A plunger 13 extends through a central opening 14 in the retainer 12, and into the chamber 7 containing the metal 10. When the liquid metal 10 cools to room temperature, it solidifies in the chamber 7.

It will be apparent from the drawing that when the plunger 13 is inserted in the cavity 7, the sealing ring 11 prevents leakage of metal 10 out of the opening 9 and the plugs 6 prevent the leakage of metal out of their respective passages 3 and 4 and the opening 5. When an axial force is applied to the plunger 13, the internal pressure in the metal 10 increases, tending to expand the volume of the cavity. As the surface area of the cavity increases due to expansion of the cylinder block 1, there is a corresponding increase in volume of the cavity. Since the metal 10 is substantially incompressible, yielding of the interior surface of the block 1 causes a decrease in the internal pressure in the metal, unless the plunger is dis placed concurrently with the yielding of the surface of the cavity 7. When the desired value of prestressing pressure is almost reached, displacement of the plunger 12 may be carried out in stepwise movements, so that the desired value of prestressing pressure will not be exceeded unintentionally.

The internal deformation of the cavity may be determined by measuring the strain on the exterior surface of the cylinder block with conventional strain gauges, or other suitable means. After the desired degree of prestressing has been accomplished, the plunger 13 is withdrawn from the cavity and the retainer 12 and the ring 11 are removed. The metal 10 then may be machined from the cavity, pressed out, or melted and poured out of the cavity. The plugs 6 are then removed from the cylinder block 1 and it is machined in accordance with conventional practice.

A nearly incompressible fluid used in this process is preferably a lead base metal such as lead and alloys of lead with bismuth and tin which may be used to provide a melting point as low as 200 F. Cadmium may also be added to the alloy to reduce the melting point to as low as 150 F. Bismuth may also be used as the fluid. Lead and bismuth and their alloys have the characteristic of being plastically deformable in the solid phase. The term plastically deformable is used herein to distinguish over elastically deformable metals. Steel, for example, is elastically deformable up to the yield point. The metals used in the process of this invention, however, are continuously plastically deformable when stressed, and are not elastically deformable. Under pressure, these metals flow with a high viscosity, although in the solid state. This is also advantageous. The seal 11 may not always be sufliciently tight to withstand the high pressure in the cavity, but if leakage around the seal 11 should occur, the flow rate of the viscous fluid would be so slow that in most instances, the process would be carried out without interruption. These metals maintain their plastically deformable characteristics under pressures as high as 20,000 atmospheres. Furthermore, these metals are substantially incompressible throughout the range of pressures encountered in carrying out this process.

One important advantage of this invention is that higher pre.stressing pressures can be obtianed than are possible with conventional fluids, such as water and oil. Another advantage of the method if this invention is that since the metal is substantially incompressible, the fluid pressure is immediately relieved upon an increase in volume of the cavity. Normally, fluids such as water or oil are used in prestressing operations. These fluids are highly compressible in comparison to the metals which are used in accordance with this invention. In the event of failure of the vessel when prestressing with a fluid, the fluid expands and forces the broken vessel outwardly, producing an explosive effect. When prestressing under the same conditions with a plastically deformable metal, there is very little compressibility to cause an explosion if the vessel should break.

The low compressibility of the metals in the solid phase which are used in the process of this invention also provides a greater degree of control over the strain imposed on the vessel. If the strain is continuously measured by strain gauges, for example, while the prestressing pressure is being applied, the pressure can be relieved when the desired degree of strain is reached. It is very difficult to control the degree of prestressing when conventional compressible fluids are used, because fluid pressure does not increase uniformly with displacement of the plunger or ram. The force on the ram is approximately proportional to the internal pressure in the substantially incompressible and plastically deformable metal of this invention, however, and therefore the degree of strain can be controlled.

While this invention has been illustrated and described in one embodiment, it is recognized that variations and changes may be made therein without departing from the invention as set forth in the claims.

We claim:

1. A method of prestressing a metal block having a cavity therein comprising filling the cavity with a substantially incompressible metal, sealing said metal in the cavity, inserting a plunger into said cavity to displace said metal, moving said plunger into said cavity with a force to develop a pressure in the metal suflicient to prestress said block, said metal being in the solid state during said plunger movement, and measuring the external strain on said block to determine the degree of prestressing.

2. The method according to claim 1 wherein said metal is lead base metal.

3. A method of prestressing a metal vessel having a cavity therein, said vessel cavity having elastically deformable walls, comprising the steps of confining a substantially incompressible plastically deformable solid in said vessel cavity, inserting a plunger into said vessel cavity to increase the pressure in said solid to a value substantially above the yield point of the vessel cavity walls, whereby prestressing of the cavity walls is accomplished without substantial deformation of said vessel walls.

4. The method according to claim 3 wherein the shape of said cavity remains substantially constant throughout said prestressing.

5. The method according to claim 3 wherein said advancing is conducted in a series of stepwise increments.

6. The method according to claim 5 including measuring the external strain on said vessel during said series of stepwise movement.

7. A method of prestressing a metal block having a cavity therein comprising filling the cavity with a substantially incompressible metal, sealing said metal in the cavity, inserting a plunger into said cavity to displace said metal, and moving said plunger into said cavity in incremental steps whereby the rate and extent of prestressing is controlled.

8. A method according to claim 7 wherein said incompressible solid is a lead base metal.

9. The method according to claim 7 wherein said metal is plastically deformable under pressure.

10. A method of prestressing a metal block having a cavity therein comprising filling the cavity with a substantially incompressible metal filling material, sealing said metal filling material in the cavity, inserting a plunger into said cavity to displace said metal filling material, moving said plunger into said cavity with a force to develop a pressure in the metal filling material sutficient to prestress said block, said metal filling material being in the solid state during said plunger movement, measuring the external strain on said block while moving said plunger into said cavity to determine the degree of prestressing, and removing the metal filling material from the cavity.

References Cited UNITED STATES PATENTS 2,111,695 3/1938 Seeber ct a1. 29-157 2,117,351 5/1938 Nordstrom 29l57.l 3,023,495 3/1962 Noland 29421 3,216,092 11/1965 Arbogast 72-58 X JOHN F. CAMPBELL, Primary Examiner.

THOMAS H. EAGER, Examiner, 

1. A METHOD OF PRESTRESSING A METAL BLOCK HAVING A CAVITY THEREIN COMPRISING FILLING THE CAVITY WITH A SUBSTANTIALLY INCOMPRESSIBLE METAL, SEALING SAID METAL IN THE CAVITY, INSERTING A PLUNGER INTO SAID CAVITY TO DISPLACE SAID METAL, MOVING SAID PLUNGER INTO SAID CAVITY WITH A FORCE 